1. Field:
The invention is in the field of detection methods and apparatus based on Schlieren optics.
2. State of the Art:
It has been known for some time that a refractive index gradient such as produced by a concentration gradient in a fluid such as gas, liquid or supercritical fluid, will cause deflection of light passing through this gradient. The optical method of observing and measuring the deflection of light caused by refractive index gradient fields is generally referred to as Schlieren optics. In most past applications, Schlieren images resulting from light deflections have been recorded on photographic plates and the plates then analyzed for light intensity distribution using densitometers. Recently, evaluation of the photographic images has been done by computer. These methods are useful in studying plasmas where very complicated toroidal and parabolic shapes are generated.
U.S. Pat. No. 4,547,071 discloses a sensor for measuring density gradients in a nonhomogenious fluid sample using Schlieren optics. In such sensor, a laser light beam is directed through a sample chamber and is moved along said chamber. A quadrant light position sensor located on the opposite side of the chamber detects the deflection of the laser light beam as it is moved through the sample. The amount of deflection indicates the density gradient at any point in the sample. Rather than moving the laser beam along the sample chamber, the beam can be held constant and the sample moved within the chamber.
While the detector of U.S. Pat. No. 4,547,071 will give an indication of the density gradients in a sample, the detector measures all density gradients equally and is not specific for any particular chemical substance. It is often desireable to be able to specifically identify a particular chemical substance. Further, laser light is not as positionally and intensity stable as is necessary for high sensitivity applications.
A current development in the field of high performance liquid chromatography is the open tubular capillary column which provide ultra high efficiency separation of sample components. This method requires very small sample volumes. However, there are currently no simple detectors available which can detect the small volumes of the separated components produced by these columns. Similar problems exist in the field of capillary zone electrophoresis where small volume samples are used. There is a need for a simple, easily used detector that can detect and identify the individual separated components of a sample of very small volume such as smaller than 100 nanoliters.